Process for Producing A Cyclic Acetal In A Heterogeneous Reaction System

ABSTRACT

A process for producing a cyclic acetal is disclosed. According to the process, a formaldehyde source is combined with an aprotic compound and contacted with a heterogeneous catalyst which causes the formaldehyde source to convert into a cyclic acetal such as trioxane. The catalyst, for instance, may comprise a solid catalyst such as an ion exchange resin. In one embodiment, the process is used for converting anhydrous formaldehyde gas to trioxane. The anhydrous formaldehyde gas may be produced form an aqueous formaldehyde solution by an extractive distillation.

BACKGROUND

1,3,5-Trioxane (hereinafter “trioxane”) is the cyclic trimer offormaldehyde. Trioxane is mainly used as a starting material for themanufacturing of polyoxymethylenes (POM) which is a high performancepolymer having desirable and exceptional properties in terms ofmechanical, chemical and temperature stability. Polyoxymethylenepolymers are available as homo- and copolymers.

As the polyoxymethylene market is growing, there is a desire on the sideof the trioxane producers to expand their production capacities in orderto satisfy the trioxane demand on a competitive basis. The majortechnical process for the production of trioxane is the conversion ofaqueous formaldehyde solutions in the presence of concentrated sulfuricacid as a catalyst. The process for the production of trioxane known inthe prior art is complex and comprises an extraction step whichnecessitates tedious solvent recovery steps. Furthermore, the processconventionally and commercially known in the prior art is time andenergy consuming and leads to a low degree of conversion of theformaldehyde source into the desired cyclic acetals. Furthermore, theamount of side products formed by the process is high.

In view of the above, a need currently exists for an efficient processfor producing cyclic acetals, such as trioxane. A need also exists for aprocess for producing cyclic acetals that has a relatively highconversion rate. A need also exists for a process for producing cyclicacetals from different formaldehyde sources.

SUMMARY

In general, the present disclosure is directed to a process forproducing one or more cyclic acetals from a formaldehyde source. Theformaldehyde source may comprise gaseous formaldehyde, paraformaldehyde,polyoxymethylene homo- and copolymers, mixtures containing formaldehydesuch as formaldehyde and trioxane mixtures, and blends thereof. Theformaldehyde source is combined with an aprotic compound and contactedwith a catalyst. In accordance with the present disclosure, the catalystis a heterogeneous catalyst. For instance, the catalyst may comprise asolid catalyst, such as an ion exchange material.

The aprotic compound may be polar. For instance, in one embodiment, theaprotic compound may be dipolar. In one embodiment, the aprotic compoundcomprises a sulfur containing organic compound such as a sulfoxide, asulfone, a sulfonate ester, or mixtures thereof. In one embodiment, theaprotic compound comprises sulfolane.

The aprotic compound may also have a relatively high static permittivityor dielectric constant of greater than about 15. The aprotic compoundmay also be nitro-group free. In particular, compounds havingnitro-groups may form undesired side reactions within the process.

Once the cyclic acetal is formed from the formaldehyde source, thecyclic acetal can be easily separated from the aprotic compound and thecatalyst. In one embodiment, for instance, the cyclic acetal may beseparated by distillation from the aprotic compound which may have amuch higher boiling point than the cyclic acetal. The aprotic compound,for instance, may have a boiling point of greater than about 120° C.,such as greater than about 140° C., such as greater than about 160° C.,such as even greater than about 180° C. at a pressure of one bar.Preferably, the aprotic compound does not form an azeotrope with thecyclic acetal.

Other features and aspects of the present disclosure are discussed ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present disclosure is set forthmore particularly in the remainder of the specification, includingreference to the accompanying figures in which:

FIG. 1 is a schematic diagram of one embodiment of a process inaccordance with the present disclosure; and

FIG. 2 is a schematic diagram of another embodiment of a process inaccordance with the present disclosure.

Repeat use of reference characters in the present specification anddrawings is intended to represent same or analogous features or elementsof the present disclosure.

DETAILED DESCRIPTION

The present disclosure is generally directed to a process for producinga cyclic acetal. Of particular advantage, cyclic acetals can be producedfrom all different types of formaldehyde sources. As used herein, aformaldehyde source includes formaldehyde and oligomers or polymersformed from formaldehyde. Thus, a formaldehyde source can includeparaformaldehyde, oxymethylene homopolymers, and oxymethylenecopolymers.

The formaldehyde source is contacted with a catalyst in the presence ofan aprotic compound to form a cyclic acetal. The aprotic compoundfacilitates production of the cyclic acetal in a manner that greatlyenhances the conversion rates. Of particular advantage, the cyclicacetal produced according to the process can then be easily separatedfrom the aprotic compound. For instance, in one embodiment, the cyclicacetal can be separated or isolated from the aprotic compound through asimple distillation process, since the aprotic compound may have a muchhigher boiling point than the cyclic acetal.

In accordance with the present disclosure, the catalyst that is usedduring conversion of the formaldehyde into the cyclic acetal comprises aheterogeneous catalyst. The catalyst, for instance, can be immiscible inthe aprotic compound and the formaldehyde source. In one embodiment, thecatalyst comprises a solid catalyst. As used herein, a solid catalyst isa catalyst that includes at least one solid component. For instance, acatalyst may comprise an acid that is adsorbed or otherwise fixed to asolid support. The catalyst may also be present in liquid phase which isnot miscible or is at least partially immiscible with the aproticcompound.

Various advantages and benefits are obtained when using a heterogeneouscatalyst. For example, when using a heterogeneous catalyst, the catalystcan be easily separated from the aprotic compound, the formaldehydesource, and/or the cyclic acetal that is produced. In addition, in oneembodiment a solid catalyst is used that remains in the reactor that isused to produce the cyclic acetal. In this manner, the catalyst can beused over and over again during the process.

Furthermore solid catalysts tend to be less corrosive to theirenvironments e.g vessel walls.

Through the process of the present disclosure, a formaldehyde source maybe converted into one or more cyclic acetals at extremely fast reactiontimes, such as within minutes. In addition, very high conversion ratescan be achieved. For instance, in one embodiment, a majority of theformaldehyde source may be converted into one or more cyclic acetals.

In one embodiment, the aprotic compound is a liquid when contacted withthe formaldehyde source. The formaldehyde source, on the other hand, maycomprise gaseous formaldehyde, a liquid, or a solid. The formaldehydesource may dissolve into the aprotic compound or may be absorbed by theaprotic compound to form a homogeneous phase. The aprotic compound andthe catalyst, in one embodiment, may comprise a liquid reaction mixtureor a liquid medium.

The formaldehyde source reacts (converts) in the presence of thecatalyst.

Cyclic acetals within the meaning of the present disclosure relate tocyclic acetals derived from formaldehyde. Typical representatives arerepresented the following formula:

wherein a is an integer ranging from 1 to 3.

Preferably, the cyclic acetals produced by the process of the presentdisclosure are trioxane (a=1) and/or tetroxane (a=2). Trioxane andtetroxane usually form the major part (at least 80 wt.-%, preferably atleast 90 wt.-%) of the cyclic acetals formed by the process of thepresent disclosure.

The weight ratio of trioxane to tetroxane varies with the heterogeneouscatalyst used. Typically, the weight ratio of trioxane to tetroxaneranges from about 3:1 to about 40:1, preferably about 4:1 to about 20:1.

The process of the invention is carried out in the presence of aheterogeneous catalyst for the conversion of the formaldehyde sourceinto cyclic acetals. Suitable catalysts are any components whichaccelerate the conversion of the formaldehyde source to the cyclicacetals. The catalyst is a catalyst for the conversion (reaction) of aformaldehyde source into cyclic acetals, preferably into trioxane and/ortetroxane.

Usually, cationic catalysts can be used for the process of theinvention. The catalysts can be a solid catalyst or an immiscible liquidcatalyst. A typical liquid immiscible catalyst is a liquid acidic ionexchange resin. Solid catalyst means that the catalyst is at leastpartly, preferably completely in solid form under the reactionconditions. Typical solid catalysts which may be used for the process ofthe present invention are acid ion-exchange material, strongly acidicion-exchange material, Lewis acids and/or Broensted acids fixed on asolid support, wherein the support may be an inorganic material such asSiO₂ or organic material such as organic polymers.

Preferred catalysts that may be fixed to a solid support are selectedfrom the group consisting of Broensted acids and Lewis acids. Thecatalyst can be selected from the group consisting oftrifluoroalkanesulfonic acids such as trifluoromethanesulfonic acid,perchloric acid, methanesulfonic acid, toluenesulfonic acid and sulfuricacid, or derivatives thereof such as anhydrides or esters or any otherderivatives that generate the corresponding acid under the reactionconditions. Lewis acids like boron trifluoride, arsenic pentafluoridecan also be used. Heteropolyacides such as tungsten heteropoly acides(e.g. tungstophosphates) may also be used. It is also possible to usemixtures of all the individual catalysts mentioned above.

In one embodiment, the heterogeneous catalyst may comprise a Lewis orBroensted acid species dissolved in an inorganic molten salt. The moltensalt may have a melting point below 200° C., such as less than about100° C., such as less than about 30° C. The molten salt can then beimmobilized or fixed onto a solid support as described above. The solidsupport, for instance, may be a polymer or a solid oxide. An example ofan organic molten salt include ionic liquids. For instance, the ionicliquid may comprise 1-n-alkyl-3-methylimidazolium triflate. Anotherexample is 1-n-alkyl-3-methylimidazolium chloride.

In one embodiment, the acidic compound present in the catalyst can havea pKa below 0, such as below about −1, such as below about −2, whenmeasured in water at a temperature of 18° C. The pKa number expressesthe strength of an acid and is related to the dissociation constant forthe acid in an aqueous solution.

Examples of heterogeneous catalysts that may be used according to thepresent disclosure include the following:

(1) solid catalysts represented by acidic metal oxide combinations whichcan be supported onto usual carrier materials such as silica, carbon,silica-alumina combinations or alumina. These metal oxide combinationscan be used as such or with inorganic or organic acid doping. Suitableexamples of this class of catalysts are amorphous silica-alumina, acidclays, such as smectites, inorganic or organic acid treated clays,pillared clays, zeolites, usually in their protonic form, and metaloxides such as ZrO₂—TiO₂ in about 1:1 molar combination and sulfatedmetal oxides e.g. sulfated ZrO₂. Other suitable examples of metal oxidecombinations, expressed in molar ratios, are: TiO₂—SiO₂ 1:1 ratio; andZrO₂—SiO₂ 1:1 ratio.

(2) several types of cation exchange resins can be used as acid catalystto carry out the reaction. Most commonly, such resins comprisecopolymers of styrene, ethylvinyl benzene and divinyl benzenefunctionalized so as to graft SO₃H groups onto the aromatic groups.These acidic resins can be used in different physical configurationssuch as in gel form, in a macro-reticulated configuration or supportedonto a carrier material such as silica or carbon or carbon nanotubes.Other types of resins include perfluorinated resins carrying carboxylicor sulfonic acid groups or both carboxylic and sulfonic acid groups.Known examples of such resins are: NAFION™, and AMBERLYST resins. Thefluorinated resins can be used as such or supported onto an inertmaterial like silica or carbon or carbon nanotubes entrapped in a highlydispersed network of metal oxides and/or silica.

(3) heterogeneous solids, having usually a lone pair of electrons, likesilica, silica-alumina combinations, alumina, zeolites, silica,activated charcoal, sand and/or silica gel can be used as support for aBroensted acid catalyst, like methane sulfonic acid or para-toluenesulfonic acid, or for a compound having a Lewis acid site, such as SbF₅,to thus interact and yield strong Broensted acidity. Heterogeneoussolids, like zeolites, silica, or mesoporous silica or polymers likee.g. polysiloxanes can be functionalized by chemical grafting with aBroensted acid group or a precursor therefore to thus yield acidicgroups like sulfonic and/or carboxylic acids or precursors therefore.The functionalization can be introduced in various ways known in the artlike: direct grafting on the solid by e.g. reaction of the SiOH groupsof the silica with chlorosulfonic acid; or can be attached to the solidby means of organic spacers which can be e.g. a perfluoro alkyl silanederivative. Broensted acid functionalized silica can also be preparedvia a sol gel process, leading to e.g. a thiol functionalized silica, byco-condensation of Si(OR)₄ and e.g. 3-mercaptopropyl-tri-methoxy silaneusing either neutral or ionic templating methods with subsequentoxidation of the thiol to the corresponding sulfonic acid by e.g. H₂O₂.The functionalized solids can be used as is, i.e. in powder form, in theform of a zeolitic membrane, or in many other ways like in admixturewith other polymers in membranes or in the form of solid extrudates orin a coating of e.g. a structural inorganic support e.g. monoliths ofcordierite; and

(4) heterogeneous heteropolyacids having most commonly the formulaH_(x)PM_(y)O_(z). In this formula, P stands for a central atom,typically silicon or phosphorus. Peripheral atoms surround the centralatom generally in a symmetrical manner. The most common peripheralelements, M, are usually Mo or W although V, Nb, and Ta are alsosuitable for that purpose. The indices xyz quantify, in a known manner,the atomic proportions in the molecule and can be determined routinely.These polyacids are found, as is well known, in many crystal forms butthe most common crystal form for the heterogeneous species is called theKeggin structure. Such heteropolyacids exhibit high thermal stabilityand are non-corrosive. The heterogeneous heteropolyacids are preferablyused on supports selected from silica gel, kieselguhr, carbon, carbonnanotubes and ion-exchange resins. A preferred heterogeneousheteropolyacid herein can be represented by the formula H₃PM₁₂O₄₀wherein M stands for W and/or Mo. Examples of preferred PM moieties canbe represented by PW₁₂, PMo₁₂, PW₁₂/SiO₂, PW₁₂/carbon and SiW₁₂.

As described above, formaldehyde or a formaldehyde source is convertedto a cyclic acetal by contacting the formaldehyde source with aproticcompound and a catalyst. The formaldehyde source, for instance, maycomprise gaseous formaldehyde. Gaseous formaldehyde can have a watercontent of less than about 5 wt-%, such as less than about 2 wt-%, suchas less than about 1 wt-%, such as less that about 0.5 wt-%. In analternative embodiment, the formaldehyde source may compriseparaformaldehyde, which can have a water content of less than 5 wt-%,such as less than 2 wt-%, such as less than about 1 wt-%.

In still another embodiment, the formaldehyde source may comprise apolyoxymethylene homo- or copolymer. The polyoxymethylene polymer canhave a molecular weight of generally greater than about 2000 Dalton. Agas or liquid stream may be fed to the reactor that containsformaldehyde in combination with other components. For instance, theformaldehyde may be present with trioxane, or other monomers used toproduce polyoxymethylene polymers. In yet another embodiment, theformaldehyde source may comprise an aqueous formaldehyde solution. Theaqueous formaldehyde solution, for instance, may contain water inamounts greater than about 30%, such as in amounts greater than about50%, such as in amounts from about 40% to about 70%.

As used herein, an aprotic compound is a compound that does not containany substantial amounts of hydrogen atoms which can disassociate. In oneembodiment, the aprotic compound is liquid under the reactionconditions. Therefore, the aprotic compound may have a melting point ofabout 180° C. or less, preferably about 150° C. or less, more preferablyabout 120° C. or less, especially about 60° C. or less.

For practical reasons, it is advantageous to use an aprotic compoundwhich has a melting point in the order of preference (the lower themelting point the more preferred) of below about 50° C., below about 40°C. and below about 30° C. and below about 20° C. Especially, aproticcompounds which are liquid at about 25 or about 30° C. are suitablesince they can be easily transported by pumps within the productionplant.

Further, the aprotic compound may have a boiling point of about 120° C.or higher, preferably about 140° C. or higher, more preferably about160° C. or higher, especially about 180° C. or higher, determined at 1bar. In a further embodiment the boiling point of the aprotic compoundis about 200° C. or higher, preferably about 230° C. or higher, morepreferably about 240° C. or higher, further preferably about 250° C. orhigher and especially about 260° C. or higher or 270° C. or higher. Thehigher the boiling point the better the cyclic acetals, especiallytrioxane and/or tetroxane, formed by the process of the presentdisclosure can be separated by distillation. Therefore, according to anespecially preferred embodiment of the present disclosure the boilingpoint of the aprotic compound is at least about 20° C. higher than theboiling point of the cyclic acetal formed, in particular at least about20° C. higher than the boiling point of trioxane and/or tetroxane.

Additionally, aprotic compounds are preferred which do not form anazeotrope with the cyclic acetal, especially do not form an azeotropewith trioxane.

In a preferred embodiment of the present invention the reaction mixtureor liquid medium in the reactor 40 comprises at least about 20 wt.-%,preferably at least about 40 wt.-%, more preferably at least about 60wt.-%, most preferably at least about 80 wt.-% and especially at leastabout 90 wt.-% of the aprotic compound(s), wherein the weight is basedon the total weight of the reaction mixture. The liquid medium or thereaction mixture or the liquid mixture (A) may comprise one or moreaprotic compound(s).

In a preferred embodiment the liquid medium is essentially consisting ofthe aprotic compound. Essentially consisting of means that the liquidmedium comprises at least about 95 wt.-%, preferably at least about 98wt.-%, more preferably at least about 99 wt.-%, especially at leastabout 99.5 wt.-%, in particular at least about 99.9 wt.-% of the aproticcompound(s). In a further embodiment of the invention the liquid mediumis the aprotic compound, i.e., the liquid medium is consisting of theaprotic compound.

It has been found that liquid aprotic compounds which at least partlydissolve or absorb the formaldehyde source lead to excellent results interms of conversion of the formaldehyde source into the desired cyclicacetals.

Therefore, aprotic compounds are preferred which at least partlydissolve or absorb the formaldehyde source under the reactionconditions. Preferred are aprotic compounds which dissolveparaformaldehyde (98 wt.-% formaldehyde, 2 wt.-% water) [can also beexpressed as Pn=moles of formaldehyde/moles ofwater=(98/30)/(2/18)=approx. 29] at the reaction temperature in anamount of at least about 0.1 wt.-%, wherein the weight is based on thetotal weight of the solution.

The aprotic compound used in the process can be a polar aproticcompound, especially a dipolar compound. Polar aprotic solvents are muchmore suitable to dissolve the formaldehyde source. Non-polar aproticcompounds such as unsubstituted hydrocarbons (e.g. cyclic hydrocarbonssuch as cyclohexane, or alicyclic hydrocarbons such as hexane, octane,decane, etc.) or unsubstituted unsaturated hydrocarbons or unsubstitutedaromatic compounds are less suitable. Therefore, according to apreferred embodiment the aprotic compound is not an unsubstitutedhydrocarbon or unsubstituted unsaturated hydrocarbon or unsubstitutedaromatic compound. Further, preferably the reaction mixture comprisesunsubstituted hydrocarbons and/or unsubstituted unsaturated hydrocarbonsand/or unsubstituted aromatic compounds in an amount of less than about50 wt.-%, more preferably less than about 25 wt.-%, further preferablyless than about 10 wt.-%, especially less than about 5 wt.-%, e.g. lessthan about 1 wt.-% or about 0 wt.-%.

Halogen containing compounds are less preferred due to environmentalaspects and due to their limited capability to dissolve the formaldehydesources. Further, the halogenated aliphatic compounds may causecorrosions in vessels or pipes of the plant and it is difficult toseparate the cyclic acetals formed from the halogenated compounds.

According to one embodiment, the aprotic compound is halogen free. In afurther preferred embodiment the reaction mixture comprises less thanabout 50 wt.-%, more preferably less than about 25 wt.-%, furtherpreferably less than 10 wt.-%, more preferably less than 5 wt.-%,especially less than 1 wt.-% or less than 50 ppm of halogenatedcompounds.

Likewise, the use of (liquid) sulphur dioxide leads to difficulties withisolation of the cyclic acetals. Therefore, the aprotic compound ispreferably free of sulphur dioxide. In a further preferred embodimentthe reaction mixture comprises less than about 50 wt.-%, more preferablyless than about 25 wt.-%, further preferably less than 10 wt.-%, morepreferably less than 5 wt.-%, especially less than 1 wt.-% or 0 wt.-% ofsulphur dioxide.

Polar aprotic compounds are especially preferred. According to apreferred embodiment of the invention the aprotic compound has arelative static permittivity of more than about 15, preferably more thanabout 16 or more than about 17, further preferably more than about 20,more preferably of more than about 25, especially of more than about 30,determined at 25° C. or in case the aprotic compound has a melting pointhigher than 25° C. the relative permittivity is determined at themelting point of the aprotic compound.

The relative static permittivity, ε_(r), can be measured for staticelectric fields as follows: first the capacitance of a test capacitorC₀, is measured with vacuum between its plates. Then, using the samecapacitor and distance between its plates the capacitance C_(x) with anaprotic compound between the plates is measured. The relative dielectricconstant can be then calculated as

$ɛ_{r} = {\frac{C_{x}}{C_{0}}.}$

Within the meaning of the present invention the relative permittivity isdetermined at 25° C. or in case the aprotic compound has a melting pointhigher than 25° C. the relative permittivity is determined at themelting point of the aprotic compound.

According to a further aspect of the invention the aprotic compound is adipolar aprotic compound.

The aprotic compound within the meaning of the present invention isgenerally a dipolar and non-protogenic compound which has a relativepermittivity as defined above of more than 15, preferably more than 25or more than 30, determined at 25° C. or in case the aprotic compoundhas a melting point higher than 25° C. the relative permittivity isdetermined at the melting point of the aprotic compound.

The process can be carried out in manner wherein the formaldehyde sourceis completely dissolved or absorbed in the liquid medium or reactionmixture or liquid mixture (A).

Therefore, according to one embodiment the formaldehyde source and theaprotic compound form a homogenous phase under the reaction conditions.Suitable aprotic compounds are selected from the group consisting oforganic sulfoxides, organic sulfones, organic sulfonate ester, andmixtures thereof.

According to a preferred embodiment the aprotic compound is selectedfrom sulfur containing organic compounds.

Further, the aprotic compound is preferably selected from the groupconsisting of cyclic or alicyclic organic sulfoxides, alicyclic orcyclic sulfones, and mixtures thereof. Excellent results can be achievedby aprotic compounds as represented by the following formula (I):

whereinn is an integer ranging from 1 to 6, preferably 2 or 3, andwherein the ring carbon atoms may optionally be substituted by one ormore substituents, preferably selected from C₁-C₈-alkyl which may bebranched or unbranched. Preferred compounds of formula (I) aresulfolane, methylsulfolane, dimethylsulfolane, ethylsulfolane,diethylsulfolane, propylsulfolane, dipropylsulfolane, butylsulfolane,dibutylsulfolane, pentylsulfolane, dipentylsulfolane, and hexylsulfolaneas well as octylsulfolane.

According to the most preferred embodiment the aprotic compound issulfolane (tetrahydrothiophene-1,1-dioxide).

Sulfolane is an excellent solvent for the formaldehyde source, it isstable under acidic conditions, it does not deactivate the catalysts andit does not form an azeotrope with trioxane. Further, it is a solventwhich is inert under the reaction conditions.

Unless indicated otherwise the expression “reaction mixture” refers tothe mixture which is used for the reaction of the formaldehyde source tothe cyclic acetals. The concentrations and amounts of the individualcomponents of the reaction mixture refer to the concentrations andamounts at the beginning of the reaction. In other words the reactionmixture is defined by the amounts of its starting materials, i.e. theamounts of initial components.

Likewise the amounts defined for the “liquid mixture” refer to theamounts of the components at the beginning of the reaction, i.e. priorto the reaction.

The formaldehyde source reacts to the cyclic acetals and, as aconsequence, the concentration of the formaldehyde source decreaseswhile the concentration of the cyclic acetals increases.

At the beginning of the reaction a typical reaction mixture of theinvention comprises a formaldehyde source which is at least partly,preferably completely dissolved or absorbed in sulfolane.

Further, an especially preferred embodiment of the present invention isa process for producing cyclic acetal comprising reacting a formaldehydesource in the presence of a catalyst wherein the reaction is carried outin sulfolane or a process for producing cyclic acetals from aformaldehyde source in the presence of a catalyst and sulfolane.

A further preferred aprotic compound is represented by formula (II):

wherein R¹ and R² are independently selected from C₁-C₈-alkyl which maybe branched or unbranched, preferably wherein R¹ and R² independentlyrepresent methyl or ethyl. Especially preferred is dimethyl sulfone.

According to a further preferred embodiment the aprotic compound isrepresented by formula (III):

whereinn is an integer ranging from 1 to 6, preferably 2 or 3, andwherein the ring carbon atoms may optionally be substituted by one ormore substituents, preferably selected from C₁-C₈-alkyl which may bebranched or unbranched.

Suitable aprotic compounds are also represented by formula (IV):

wherein R³ and R⁴ are independently selected from C₁-C₈-alkyl which maybe branched or unbranched, preferably wherein R¹ and R² independentlyrepresent methyl or ethyl.

Especially preferred is dimethyl sulfoxide.

Suitable aprotic compounds may be selected from aliphatic dinitriles,preferably adiponitrile.

In a further aspect of the invention a mixture of two or more aproticcompounds is used. A mixture of aprotic compounds may be used todecrease the melting point of the aprotic medium. In a preferredembodiment the aprotic compound comprises or is consisting of a mixtureof sulfolane and dimethyl sulfoxide.

Advantageously, the aprotic compound does not essentially deactivate thecatalyst. Preferably, under the reaction conditions the aprotic compounddoes essentially not deactivate the catalyst used in the process of thepresent invention. Aprotic solvents such as dimethylformamide (DMF),dimethylacetamide (DMAC) or N-methylpyrrolidone (NMP) are too basic andtherefore may deactivate the catalyst and, as a consequence, saidsolvents are less suitable. According to a preferred embodiment of thepresent invention the liquid reaction mixture is essentially free ofamides, preferably essentially free of acylic or cyclic amides.Essentially free means that the amides may be present in an amount ofless than about 5 wt.-%, preferably less than about 2 wt.-%, morepreferably less than 0.5 wt.-%, especially less than about 0.01 wt.-%and, in particular, less than 0.001 wt.-% or about 0 wt.-%, wherein theweight is based on the total weight of the liquid reaction mixture.

Nitro group containing compounds can lead to undesired side products oreven demonstrate an insufficient solubility for the formaldehydesources.

Therefore, the aprotic compound preferably does not comprise a nitrogroup and/or a nitrogen atom. Further, according to a preferredembodiment of the present invention the aprotic compound is anon-aromatic aprotic compound. Especially, the aprotic compound is notnitrobenzene or an aromatic nitro compound. Further, preferably, theaprotic compound does not comprise ether.

Within the meaning of the present invention the aprotic compound doesnot deactivate the catalyst if under the reaction conditions less thanabout 95%, preferably less than about 50%, more preferably less thanabout 10%, of the Broensted acid catalyst used protonates the aproticcompound. In case a Lewis acid catalyst is used the aprotic compounddoes not deactivate the catalyst if under the reaction conditions lessthan about 90 wt-%, preferably less than about 50 wt.-%, more preferablyless than about 10 wt-% of the Lewis acid catalyst forms a complex withthe aprotic compound.

The degree of protonation and complex formation can be determined by NMRspectroscopy such as ¹H or ¹³C-NMR. The degree of protonation andcomplex formation is determined at 250° C., preferably in d₆-DMSO.

The deactivation of the catalyst can also be determined in the followingmanner:

10 g of commercially available paraformaldehyde (95 wt-%) is dissolvedin 100 g of sulfolane at a temperature sufficient to dissolve theparaformaldehyde in such a way that no gaseous formaldehyde can escape.The clear solution is kept at 90° C. and 0.1 wt-% of triflic acid isadded. The rate of the formation of trioxane is measured (by measuringthe concentration of trioxane as a function of time).

The same experiment is repeated, except that 10 g of the sulfolane arereplaced by 10 g of the aprotic compound to be tested. If the rate oftrioxane formation is still greater than about 1%, preferably greaterthan about 5%, more preferably greater than about 10%, of the rate ofthe initial experiment then it is concluded that the aprotic compound inquestion does not deactivate the catalyst (even though it may reduce itsactivity).

The aprotic compound should not be too basic in order to avoiddeactivation of the catalysts. On the other hand the aprotic compoundpreferably does not chemically react with the formaldehyde source underthe reaction conditions, i.e. is an inert aprotic compound.

Preferably, under the reaction conditions the aprotic compound shouldnot react chemically with the formaldehyde source or the cyclic acetalobtained by the process of the invention. Compounds like water andalcohols are not suitable as they react with formaldehyde. Within themeaning of the present invention an aprotic compound does not chemicallyreact with the formaldehyde source when it meets the following testcriteria:

5 g of commercially available paraformaldehyde (95 wt.-%) is added to100 g of the aprotic compound containing 0.1 wt.-%trifluoromethanesulfonic acid and heated at 120° C. for 1 hour withstirring in a closed vessel so that no gaseous formaldehyde can escape.If less than about 1 wt.-%, preferably less than about 0.5 wt.-%, morepreferably less than about 0.1 wt.-% and most preferably less than about0.01 wt.-% of the aprotic compound has chemically reacted, then theaprotic compound is considered not to have reacted with the formaldehydesource. If the aprotic compound meets the criteria it is consideredinert.

Further, under the acidic reaction conditions the aprotic compoundshould be essentially stable. Therefore, aliphatic ethers or acetals areless suitable as aprotic compounds. The aprotic compound is consideredstable under acidic conditions within the meaning of the presentinvention if the aprotic compound meets the following test conditions:

100 g of the aprotic compound to be tested containing 0.5% by weight(wt.-%) trifluoromethanesulfonic acid is heated at 120° C. for 1 hour.If less than about 0.5 wt.-%, preferably less than about 0.05 wt.-%,more preferably less than about 0.01 wt.-% and most preferably less thanabout 0.001 wt.-% of the aprotic compound has chemically reacted, thenthe aprotic compound is considered to be stable under acidic conditions.

The process of the invention can also be used to change the ratio ofcyclic acetals derived from formaldehyde. Therefore, the formaldehydesource can also comprise cyclic acetals selected from the groupconsisting of trioxane, tetroxane and cyclic oligomers derived fromformaldehyde.

Preferably, the reaction mixture comprises the formaldehyde source in anamount ranging from about 0.1 to about 60 wt-% or about 1 to less thanabout 30 wt.-%, more preferably from about 5 to about 15 wt-%, furtherpreferably ranging from about 7 to about 13 wt-% and most preferredranging from about 8 to about 12 wt-%, especially ranging from 30 to 60wt.-% based on the total weight of the reaction mixture.

It has been found that especially good results in terms of conversioncan be achieved when the weight ration of formaldehyde/water of theformaldehyde source is greater than 4, preferable greater than 10 mostpreferably greater than 20.

Typically, the reaction is carried out at a temperature higher thanabout 0° C., preferably ranging from about 30° C. to about 170° C., morepreferably ranging from about 40° C. to about 140° C., furtherpreferably from about 40° C. to about 120° C. and most preferably fromabout 50° C. to about 110° C.

The pressure during the reaction can generally be from about 10millibars to about 20 bars, such as from about 0.5 bar to about 10 bar,such as from about 0.5 bar to about 2 bar.

A further advantage of the process of the present invention is that thecyclic acetals can easily be separated from the reaction mixture. Thecyclic acetal, especially the trioxane can be separated from thereaction mixture by distillation in a high purity grade. Especially incase aprotic compounds (such as sulfolane) having a boiling point higherthan about 20° C. above the boiling point of the cyclic acetals is usedthe formed cyclic acetals can simply be distilled off. In case sulfolaneis used as the aprotic compound the formed trioxane can be distilled offwithout the formation of an azeotrope of sulfolane with trioxane. Theprocess of the invention can be carried out batch wise or as acontinuous process.

In a preferred embodiment the process is carried out as a continuousprocess wherein the formaldehyde source is continuously fed to theliquid medium comprising the catalyst and wherein the cyclic acetals,e.g. the trioxane, is continuously separated (isolated) by separationmethods such as distillation.

The process of the invention leads to an extremely high conversion ofthe formaldehyde source to the desired cyclic acetals.

According to a preferred embodiment the final conversion of theformaldehyde source to the cyclic acetal is greater than 10%, based oninitial formaldehyde source.

The final conversion refers to the conversion of the formaldehyde sourceinto the cyclic acetals in the liquid system. The final conversioncorresponds to the maximum conversion achieved in the liquid system.

The final conversion of the formaldehyde source to the cyclic acetalscan be calculated by dividing the amount of cyclic acetals (expressed inwt.-%, based on the total weight of the reaction mixture) in thereaction mixture at the end of the reaction divided by the amount offormaldehyde source (expressed in wt.-%, based on the total weight ofthe reaction mixture) at the beginning of the reaction at t=0.

For example the final conversion of the formaldehyde source to trioxanecan be calculated as:

Final conversion=(amount of trioxane in the reaction mixture expressedin weight-% at the end of the reaction)/(amount of formaldehyde sourcein the reaction mixture expressed in weight-% at t=0 [initial amount offormaldehyde source in the reaction mixture]).

According to a further preferred embodiment of the process of theinvention the final conversion of the formaldehyde source into thecyclic acetals, preferably trioxane and/or tetroxane, is higher than12%, preferably higher than 14%, more preferably higher than 16%,further preferably higher than 20%, especially higher than 30%,particularly higher than 50%, for example higher than 80% or higher than90%.

The process for producing cyclic acetals in accordance with the presentdisclosure can be conducted continuously or can be conducted in abatch-wise manner (discontinuous). Referring to FIG. 1, one embodimentof a continuous process for producing a cyclic acetal in accordance withthe present disclosure is shown. The process illustrated in FIG. 1 isparticularly well suited for converting anhydrous formaldehyde gas intoa cyclic acetal such as trioxane. It should be understood, however, thatthe process in FIG. 1 may also be used to process any of theformaldehyde sources described above.

Referring to FIG. 1, the process includes an inlet stream 10 for feedinga formaldehyde source to a first fixed bed reactor 12. In oneembodiment, the inlet 10 is for feeding a gaseous formaldehyde, or agaseous/paraformaldehyde fluid stream to the reactor 12. In addition tothe inlet stream 10, the process also includes aprotic compound stream14 for feeding an aprotic compound to the reactor 12 in combination withthe formaldehyde source. The aprotic compound fed to the reactor, forinstance, may comprise liquid sulfolane. In one embodiment, forinstance, a liquid aprotic compound is fed to the reactor that is notheated and is at a temperature of less than about 50° C., such as lessthan about 40° C., such as from about 15° C. to about 25° C.

The fixed bed reactor 12, in one embodiment, can contain a solidcatalyst. The catalyst bed can be placed above, below, or in betweeninert materials, such as solid oxides, or a mixture of solid oxides. Theinert materials may improve the radial distribution of the gas/liquidstream and avoid loss of catalyst. Use of the inert material, however,is optional.

In one embodiment, the fixed bed reactor 12 is operated as acontinuous-liquid trickle bed reactor. For instance, the gas and theliquid velocity can be selected such that a trickle flow regime or apulsating regime is achieved. Superficial liquid velocities can bebetween about 5 m/hr to about 20 m/hr, such as from about 15 m/hr toabout 100 m/hr. The liquid-gas mass ratio at the reactor inlet can bebetween about 2 kg/kg to about 30 kg/kg, such as from about 5 kg/kg toabout 10 kg/kg.

The temperature within the reactor 12 can be from about 30° C. to about200° C., such as from about 80 C to about 120° C.

The pressure within the reactor 12 can generally be from about 0.15 barto about 5 bar (absolute), such as from about 1 bar to about 2 bar.

Within the fixed bed reactor 12, the formaldehyde source is convertedinto a cyclic acetal. A gas/liquid stream 15 is produced that is thenfed to a gas/liquid flash drum 18. Within the flash drum 18, agas-liquid separation takes place. In addition to a flash drum, theprocess may also include an integrated gas-liquid calming zone withinthe reactor.

From the flash drum 18, a hot liquid outlet stream 22 is produced thatis then fed to a heat exchanger 20. The heat exchanger 20 can remove theheat of dissolution and the heat of reaction. A vapor/unconvertedformaldehyde stream 26 is also produced by the flash drum 18.

As shown in FIG. 1, in one embodiment, the hot liquid outlet stream 22,which contains the cyclic acetal can be split into a first liquid stream28 and a second liquid steam 16. The second liquid stream 16 comprises arecycled liquid product stream that is fed back to the first fixed bedreactor 12. The liquid stream 28, on the other hand, is fed to a secondfixed bed reactor 24. In addition, the vapor stream 26 is also fed tothe second fixed bed reactor 24. The second fixed bed reactor 24 canoperated similar to the first fixed bed reactor 12. The second fixed bedreactor is a “polishing” reactor that is designed to further increaseconversion of the formaldehyde source into the cyclic acetal. Throughthe process shown in FIG. 1, for instance, formaldehyde conversion tothe cyclic acetal can be greater than about 50%, such as greater thanabout 70%, such as greater than about 90%. In one embodiment, forinstance, more than 95%, such as more than 98%, such as even more than99% of the formaldehyde source may be converted into a cyclic acetal.

The second fixed bed reactor 24, produces a gas/liquid outlet stream 30that is then fed to a second flash drum 32. The flash drum 32 producesan off gas stream 36 and a product stream 34. The product stream 34contains the cyclic acetal, such as trioxane, the aprotic compound,water and formaldehyde. The product stream 34 can be fed to adistillation process for separating and removing the cyclic acetal. Theaprotic compound can also be separated and fed back to the first fixedbed reactor 12.

Referring to FIG. 2, an alternative embodiment of a process inaccordance with the present disclosure is shown. In the embodimentillustrated in FIG. 2, instead of a fixed bed reactor, the processincludes a suspended reaction system. In particular, as shown in FIG. 2,the process includes a stirred tank reactor 50 that contains a suspendedsolid catalyst material. The catalyst concentration within the reactorcan be less than about 75% by weight, such as less than about 50% byweight, such as less than about 50% by weight, such as less than about3% by weight. The suspended catalyst is retained by means of filtrationinside the reactor or by means of cross flow filtration outside thereactor. A formaldehyde source is fed to the reactor 50 through an input52. The formaldehyde source, which may comprise formaldehyde gas, isdispersed within the reactor by means of stirring. The stirring powerinput to the system can be from about 0.01 kW/m³, to about 20 kW/m³,such as from about 0.1 kW/m³, to about 3 kW/m³.

In addition to a formaldehyde source, an aprotic compound is also fed tothe reactor 50 through the aprotic compound feed line 54. The reactor 50produces a liquid product stream 58 and an outgas stream 56. The reactor50 can operate generally at the same pressures and temperatures asdescribed above. The liquid product stream 58 containing a cyclic acetalis fed to a heat exchanger 60 for removing heat. In one embodiment, theliquid product stream 58 can be divided into a recycle stream 64 that isfed back to the reactor 50 and a product stream 62. The product stream62 can contain primarily the liquid aprotic compound and the cyclicacetal such as trioxane. The liquid product stream 62 can be fed to adistillation process for removing and separating the trioxane. Theaprotic compound can then be fed back to the reactor.

The present disclosure may be better understood with reference to thefollowing example.

Cyclic acetals made according to the present disclosure can be used innumerous and diverse applications. In one embodiment, for instance, thecyclic acetals produced by the present disclosure may be used to producean oxymethylene polymer.

The oxymethylene polymer production process may comprise any suitableprocess for producing oxymethylene homopolymers and/or copolymers. Thepolymer production process, for instance, may comprise an anionicpolymerization process or a cationic polymerization process. The processfor producing the oxymethylene polymer may comprise a heterogeneousprocess where the polymer precipitates in a liquid, may comprise ahomogeneous process such as a bulk polymerization process that forms amolten polymer or may be a polymer process that includes both aheterogeneous phase and a homogeneous phase.

For the preparation of oxymethylene polymers, a monomer that forms—CH₂—O— units or a mixture of different monomers, are reacted in thepresence of an initiator. Examples of monomers that form —CH₂O— unitsare formaldehyde or its cyclic oligomers, such as1,3,5-trioxane(trioxane) or 1,3,5,7-tetraoxocane.

The oxymethylene polymers are generally unbranched linear polymers whichgenerally contain at least 80 mol %, preferably at least 90 mol %, inparticular at least 95 mol %, of oxymethylene units (—CH₂—O—). Alongsidethese, the oxymethylene polymers contain —(CH₂)x-O— units, where x canassume the values from 2 to 25. Small amounts of branching agents can beused if desired. Examples of branching agents used are alcohols whosefunctionality is three or higher, or their derivatives, preferably tri-to hexahydric alcohols or their derivatives. Preferred derivatives areformulas in which, respectively, two OH groups have been reacted withformaldehyde, other branching agents include monofunctional and/orpolyfunctional glycidyl compounds, such as glycidyl ethers. The amountof branching agents is usually not more than 1% by weight, based on thetotal amount of monomer used for the preparation of the oxymethylenepolymers, preferably not more than 0.3% by weight.

Oxymethylene polymers can also contain hydroxyalkylene end groups—O—(CH₂₎ _(x)—OH, alongside methoxy end groups, where x can assume thevalues from 2 to 25. These polymers can be prepared by carrying out thepolymerization in the presence of diols of the general formula HO—(CH₂₎_(x)—OH, where x can assume the values from 2 to 25. The polymerizationin the presence of the diols leads, via chain transfer, to polymershaving hydroxyalkylene end groups. The concentration of the diols in thereaction mixture depends on the percentage of the end groups intended tobe present in the form of —O—(CH₂)_(x)—OH, and is from 10 ppm by weightto 2 percent by weight.

The molecular weights of these polymers, expressed via the volume meltindex MVR, can be adjusted within a wide range. The polymers typicallyhave repeat structural units of the formula —(CH₂—O—)_(n)—, where nindicates the average degree of polymerization (number average) andpreferably varies in the range from 100 to 10 000, in particular from500 to 4000.

Oxymethylene polymers can be prepared in which at least 80%, preferablyat least 90%, particularly preferably at least 95%, of all of the endgroups are alkyl ether groups, in particular methoxy or ethoxy groups.

Comonomers that may be used to produce oxymethylene copolymers includingcyclic ethers or cyclic formals. Examples include, for instance,1,3-dioxolane, diethylene glycol formal, 1,4-butanediol formal, ethyleneoxide, propylene 1,2-oxide, butylene 1,2-oxide, butylene 1,3-oxide, 1,3dioxane, 1,3,6-trioxocane, and the like. In general, one or more of theabove comonomers may be present in an amount from about 0.1 to about 20mol %, such as from about 0.2 to about 10 mol %, based on the amount oftrioxane.

The molecular weight of the resultant homo- and copolymers can beadjusted via use of acetals of formaldehyde (chain transfer agents).These also lead to production of etherified end groups of the polymers,and a separate reaction with capping reagents can therefore be omitted.Chain transfer agents used are monomeric or oligomeric acetals offormaldehyde. Preferred chain transfer agents are compounds of theformula I

R¹—(O—CH₂)_(q)—O—R²  (I),

in which R¹ and R², independently of one another, are monovalent organicradicals, preferably alkyl radicals, such as butyl, propyl, ethyl, andin particular methyl, and q is a whole number from 1 to 50.

Particularly preferred chain transfer agents are compounds of theformula I, in which q=1, very particularly preferably methylal.

The amounts used of the chain transfer agents are usually up to 5000ppm, preferably from 100 to 3000 ppm, based on the monomer (mixture).

The present disclosure may be better understood with reference to thefollowing example.

EXAMPLE 1

In this example, a strongly acidic ion exchange resin (Amberlyst 15®,wet form, from DOW CHEMICAL) was used as the catalyst. Before use, theresin was conditioned to sulfolane (exchange of water in the pores ofthe resin by sulfolane).

9 g of commercial paraformaldehyde with a water content of ca. 4 wt-%were added to 91 g of sulfolane at 145° C. with stirring. As theparaformaldehyde dissolves the temperature decreases to 122° C. Theclear solution was allowed to cool to 100° C. At that temperature 10 gof Amberlyst 15® was added. After 10 min at 100° C. the reaction mixturewas allowed to cool to 50° C., and no precipitate formed, indicating theconversion of the paraformaldehyde to trioxane. The concentration of thetrioxane in the reaction mixture is estimated to be above 6 wt-%.

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood that aspects of the various embodiments may beinterchanged both in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in such appended claims.

1. A process for producing a cyclic acetal comprising: contacting aformaldehyde source with a liquid medium comprising a liquid aproticcompound in the presence of a heterogeneous catalyst, the heterogeneouscatalyst comprising a solid catalyst; and at least partially convertingthe formaldehyde source into a cyclic acetal.
 2. A process as defined inclaim 1, wherein the heterogeneous catalyst comprises an acidion-exchange material.
 3. A process as defined in claim 1, wherein theheterogeneous catalyst comprises a Lewis acid or a Broensted acid fixedon a solid support.
 4. A process as defined in claim 3, wherein thesolid support is an inorganic material.
 5. A process as defined in claim3, wherein the solid support is an organic material comprising apolymer.
 6. A process as defined in claim 1, wherein the formaldehydesource comprises gaseous formaldehyde.
 7. A process as defined in claim1, wherein the formaldehyde source comprises a paraformaldehyde,polyoxymethylene homo-or compolymer, an aqueous formaldehyde solution,or mixtures thereof.
 8. A process as defined in claim 1, wherein theformaldehyde source comprises a mixture of formaldehyde with trioxane.9. A process as defined in claim 1, where in the heterogeneous catalystcomprises a Lewis acid or Broensted acid dissolved in an organic moltensalt immobilized on a solid support.
 10. (canceled)
 11. A process asdefined in claim 1, wherein the formaldehyde source is converted intothe cyclic acetal at a temperature of from about 80° C. to about 120°C., and at a pressure from about 0.15 bar to about 5 bar.
 12. A processas defined in claim 1, wherein the heterogeneous catalyst is containedin a fixed bed and wherein the formaldehyde source and liquid mediumflow through the fixed bed at a superficial liquid velocity of fromabout 5 m/hr to about 200 m/hr.
 13. A process as defined in claim 12,wherein the formaldehyde source is converted into a cyclic acetal in areactor that has an inlet into which the formaldehyde source and liquidmedium is fed, and wherein the reactor has a liquid-gas mass ratio atthe reactor inlet of from about 2 kg/kg to about 30 kg/kg.
 14. A processas defined in claim 1, wherein the formaldehyde source is converted intothe cyclic acetal in an agitated tank reactor, the heterogeneouscatalyst being suspended within the tank reactor.
 15. A processaccording to claim 1, wherein the aprotic compound is a polar aproticcompound.
 16. A process according to claim 1, wherein the aproticcompound has a boiling point of 140° C. or higher, determined at 1 bar.17. A process according to claim 1 wherein the aprotic compound has arelative static permittivity of more than 15, determined at 25° C.
 18. Aprocess according to claim 1 wherein higher than 50%, of theformaldehyde source is converted into one or more cyclic acetals duringthe reaction.
 19. A process according to claim 1 wherein the aproticcompound comprises a sulfur-containing organic compound.
 20. A processaccording to claim 1 wherein the aprotic compound is represented byformula (I):

wherein n is an integer ranging from 1 to 6, and wherein the ring carbonatoms may optionally be substituted by one or more substituents,preferably selected from C₁-C₈-alkyl which may be branched orunbranched.
 21. A process according to claim 1 wherein the aproticcompound is sulfolane.
 22. A process according to claim 1 wherein theaprotic compound is represented by formula (II):

wherein R¹ and R² are independently selected from C₁-C₈-alkyl which maybe branched or unbranched.
 23. A process according to claim 1 whereinthe aprotic compound is represented by formula (III):

wherein n is an integer ranging from 1 to 6, and wherein the ring carbonatoms may optionally be substituted by one or more substituents,selected from C₁-C₈-alkyl which may be branched or unbranched; or theaprotic compound is represented by formula (IV):

wherein R³ and R⁴ are independently selected from C₁-C₈-alkyl which maybe branched or unbranched.
 24. A process according to claim 1, furthercomprising the step of separating the cyclic acetal from the liquidmedium by distillation.
 25. A process according to claim 1, furthercomprising the step of manufacturing polyoxymethylene from the cyclicacetal.